There are many methods for biochemical characterisation of cells and tissues. Methods such as electrophoresis, chromatography, mass spectrometry, microarrays, etc. are used to analyse the molecular composition of cells or tissues. The results of such analyses may indicate a disease state, for example. Analyses are most often carried out after lysing cells to release their contents, and it is usually necessary to use a large number of cells, because it is difficult to isolate single cells and because normal methods of detection are not sensitive enough to measure the contents of single cells.
It is rare, however, to find a living system comprising cells that are all in the same state: cell cultures artificially synchronised in the laboratory may approach homogeneity, but cells even of the same type in a natural situation will be in different states e.g. at different stages in the cell cycle, etc. Typical analyses thus represent an average of cells being analysed.
For a more complete description of the state of any system, it would be advantageous to analyse individual cells. For example, many disease states in humans elicit changes to the white blood cells, and in Hodgkin's lymphoma it has been shown that the gene expression pattern of individual lymphocytes is not representative of the population as a whole [1]. Analysis of a mixture of cells thus masks heterogeneity within the mixture, and fails to provide information which is likely to be important for understanding the disease state. Subtle but important variations between cells are lost within experimental noise.
There are many examples in biology and medicine where analysis of individual cells would be more useful than analysis of a complete population or collection. It is a major objective of developmental biology to have a description of the molecular changes that accompany growth and differentiation of an organism. Embryological studies by definition begin with a single cell. Processes in living cells are organised into systems that respond to stimuli: to study such systems and their controls it is necessary to measure levels of molecules involved—mRNAs, proteins, metabolites etc.—in a number of cells. Disease states are often reflected in the composition of cells and tissues. Cancer cells differ from their normal counterparts in the genes that are expressed at the mRNA and protein levels. Fetal blood cells can escape into maternal circulation and must be analysed separately from the maternal cells. Autoimmune and infectious diseases result in changes to the composition of the white blood cells. Circulating white blood cells are themselves heterogeneous and comprise several different functional types, including neutrophils, lymphocytes, monocytes and platelets. A description of the molecular compositions of such collections of cells will advance basic understanding of biological systems and processes and can inform research into the causes and treatment of disease.
Reference 2 coined the term “chemical cytometry” to describe the use of high-sensitivity chemical analysis techniques to study single cells, and reference 3 reviews basic features of single cell analysis. Reference 4 reviews microtechnologies and nanotechnologies for single-cell analyses. Reference 5 describes microfluidic devices for manipulating single cells. Single cell isolation apparatuses are disclosed in references 6 & 7.
It is an object of the invention to provide further and improved devices and processes for analysing individual cells, and in particular their genomes, transcriptomes and proteomes.